Is it bad to use subprocess.call() or os.system() when writing Python shell scripts?

Question

I was researching on whether or not Python can replace Bash for shell scripting purposes. I have seen that Python can execute Linux commands using subprocess.call() or os.system(). But I've read somewhere (forgot the link of the article) that using these is a bad thing. Is this really true?

If yes, then why is it a bad thing?

If not, then is it safe to say that Python can indeed replace Bash for scripting since I could just execute Linux commands using either of the 2 function calls?

Note: If I'm not mistaken, os.system() is deprecated and subprocess.call() should be used instead but that is not the main point of the question.

Answer 1

Using os.system() or subprocess.call(..., shell=True) can't replace a shell, because they actually use the shell.

os.system("foo") actually invokes sh -c "foo" -- that is to say, it runs foo as a shell script. Using this, then, is in no respect replacing a shell. This is also true in the exact same way for subprocess.call("foo", shell=True).

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Using subprocess.Popen and functions from that family can replace a shell, but this often results in verbose and unwieldy code.

Consider the following shell script:

#!/bin/sh
foo "$1" | bar "$2"

Now, let's look at what it takes to reproduce that in Python in a way that doesn't start any shell under-the-hood:

#!/usr/bin/env python
import subprocess, sys

p1 = subprocess.Popen(["foo", sys.argv[1]], stdout=subprocess.PIPE)
p2 = subprocess.Popen(["bar", sys.argv[2]], stdin=p1.stdout)
sys.exit(p2.wait())

We went from 19 characters (after the shebang) to 148 characters (after the shebang and imports) -- and this was for a completely trivial script, one not using fancier features such as process substitution, command substitution, or the like.